A wireless radio network may provide a communication link between callers on the Public Switch Telephone Network (PSTN) and callers on mobile stations (MSs), i.e., cellular phones. One exemplary wireless radio network is a Code Device Multiple Access (CDMA) cellular communication system. The PSTN supports Pulse Code Modulated (PCM) speech signals, which are digital speech signals sampled at a frequency of 8 KHz. The CDMA network comprises a Base Station Controller (BSC) for compressing groups of 160 PCM speech samples from the PSTN into 20 ms voice coded (vocoded) frames, and a Radio Base Station (RBS) for modulating the vocoded frames into spread-spectrum signals and broadcasting the spread-spectrum signals to the MSs.
The BSC further comprises processor boards for processing each speech channel of the BSC. The processing includes compressing PCM speech samples from the PSTN into 20 ms vocoded frames, and performing echo cancellation and radio management functions for each speech channel of the BSC.
In first and second generation BSCs, timing information is distributed to the processor boards from a timing server. The timing information is used to time the operations of the processor boards. Typically, the timing server is connected to a Global Positioning System (GPS) receiver to provide the processor boards with timing information based on “GPS time”. This is done because the operations of the RBS and the MSs are both typically disciplined to “GPS time” to provide highly accurate timing between the RBS and the MSs.
FIG. 5 shows a timing distribution network for a BSC used in first and second generation CDMA networks. The timing network comprises a primary timing server 510, a secondary timing server 512, a first GPS receiver 515a connected to the primary timing server 510, and a second GPS receiver 515b connected to the secondary timing server 512. Each timing server 510 and 512 typically outputs a 50 Hz clock signal based on “GPS time” provided from its GPS receiver 515a and 515b. The timing network further comprises a network switch 522 connected to the output of each timing server 510 and 512 via cables 517, typically twisted pair RS-422 cables. The network switch 522 is also connected to processor boards 530a–530c in the BSC via a cable 527. The network switch 522 passes the clock signal from one of the two timing servers 510 and 512 to the processor boards 530a–530c. The timing network further comprises a network manager 525 for switching the network switch 522 between the clock signal from the primary timing server 510 and the clock signal from the secondary timing server 512.
During normal operation, the network switch 522 passes the clock signal from the primary timing server 510 to the processor boards 530a–530c. The processor boards 530a–530c use the received clock signal to time their operations. The network manager 525 monitors the primary timing server 510 to make sure that it is operating properly. The network manager 525 may do this by monitoring the toggling of a clock signal in the primary timing server 510, the stability of a Phase Lock Loop (PLL) in the primary timing server 510, and the availability of the GPS receiver 515a connected to the primary timing server 510. When the network manager 525 detects that the primary timing server 510 is not operating properly, the network manager 525 switches the network switch 522 over to the secondary timing server 512 so that the clock signal from the secondary timing server 512 is passed to the processor boards 530a–530c. Thus, the timing network responds to a failure of the primary timing server 510 by switching over to the secondary timing server 512, which acts as a backup timing server for the timing network.
A problem with this timing network is that it relies on proper operation of the network manager 525 for switching over from the primary timing server 510 to the secondary timing server 512 when the primary timing server 510 fails. However, the network manager 525 is not itself fault tolerant, thereby reducing the reliability of the timing network. In addition, the network manager 525 needs to monitor the operations of the primary timing server 510, adding both to the cost and complexity of the timing network. Furthermore, this timing network does not address potential transport errors of the clock signals in traveling from the timing servers 510 and 512 to the processor boards 530a–530c which may be due to a fault network switch 522.